Spontaneous Reduction by One Electron on Water Microdroplets Facilitates Direct Carboxylation with CO₂

Abstract

Recent advances in microdroplet chemistry have shown that chemical reactions in water microdroplets can be accelerated by several orders of magnitude compared to the same reactions in bulk water. Among the large plethora of unique properties of microdroplets, an especially intriguing one is the strong reducing power that can be sometimes as high as alkali metals as a result of the spontaneously generated electrons. In this study, we design a catalyst-free strategy that takes advantage of the reducing ability of water microdroplets to reduce a certain molecule, and the reduced form of that molecule can convert CO 2 into value-added products. By spraying the water solution of C 6 F 5 I into microdroplets, an exotic and fragile radical anion, C 6 F 5 I •− , is observed, where the excess electron counter-intuitively locates on the σ* antibonding orbital of the C−I bond as evidenced by anion photoelectron spectroscopy. This electron weakens the C−I bond and causes the formation of C 6 F 5 − , and the latter attacks the carbon atom on CO 2 , forming the pentafluorobenzoate product, C 6 F 5 CO 2 − . This study provides a good example of strategically making use of the spontaneous properties of water microdroplets, and we anticipate that microdroplet chemistry will be a green avenue rich in new opportunities in CO 2 utilization.

Molecular Anions Perspective

“…It turns out that in a recent collaboration Kit Bowen's group has noticed 35 generated in the sprayed water droplets in higher than expected (in bulk) concentrations, much like Zare proposed in ref 32.…”

“…It turns out that in a recent collaboration Kit Bowen’s group has noticed that sprayed water microdroplets containing CO 2 and I–C 6 F 5 can cleave the I–C σ bond to produce C 6 F 5 – , which subsequently reacts with CO 2 to generate F 5 C 5 CO 2 – which they then detect. These authors suggest that free electrons are generated in the sprayed water droplets in higher than expected (in bulk) concentrations, much like Zare proposed in ref .…”

Spontaneous and Simultaneous Oxidation and Reduction of o-Quinones in Water Microdroplets

“…In the recent several years, water microdroplet chemistry is emerging as an exciting new field for the abilities to accelerate chemical reactions by several orders of magnitude compared to the same reactions in bulk water and to trigger spontaneous reactions that cannot occur in bulk water. , Among the unique properties of water microdroplets, the observation that water microdroplets exhibit strong reducing and oxidizing powers is especially intriguing. − It is been postulated that there is a high electric field (∼10 9 V/m) at the surface of microdroplets either due to the formation of electric double layers or the alignment of the free O–H bonds of water molecules. − This high field can even pull an electron out of a hydroxide ion, − forming a hydroxyl radical and an electron (Figure ). The strong reducing power of water microdroplets is considered to be a result of the electron, and the oxidizing power of water microdroplets is a result of the hydroxyl radical.…”

“…It is worth mentioning that on the aerial surface of air bubbles in water, the oxidation potential of OH – to OH was found to occur at least 0.7 V below the redox table values as measured by electrochemical methods, so the reaction in Figure should be easier at the air–water interface compared to the bulk. The power of water microdroplets to promote reduction chemistry has been demonstrated in the formation of pyridyl anion in spraying of an aqueous solution containing dissolved pyridine, the reduction of dissolved chloroauric acid in water microdroplets to yield gold nanoparticles and nanowires, the reduction of doubly charged viologen compounds to singly charged viologen, the reduction of organic compounds by hydrogenation, the reduction of CO 2 into formate, , the reduction of Hg 2+ into atomic Hg, the reduction of C 6 F 5 I into C 6 F 5 I •– and the subsequent C–I bond cleavage, and the reduction of Fe 3+ and Cu 2+ complexes into Fe 2+ and Cu + complexes . In most of these cases, the reduction reactions were finished by solely the electrons, but some others needed the participation of other species such as protons in water.…”

“…Since the electron and the hydroxyl radical coexist as shown in Figure , water microdroplets should presumably exhibit reducing and oxidizing powers simultaneously. However, most of the existing publications show either only reduction reactions − or only oxidation reactions. − This phenomenon might be a result of the redox properties of the substrates per se , since some compounds are easy to oxidize but difficult to reduce and some compounds are easy to reduce but difficult to oxidize. There have been only two cases , where reduction and oxidation reactions occurred simultaneously to the same substrate.…”